The present invention relates generally to micro-electro-mechanical systems (MEMS). More specifically, a compact high aspect ratio MEMS device is disclosed.
MEMS optical mirror arrays have a variety of applications. A two-dimensional MEMS scanning mirror array may be used as an optical switch, for image projection, or for other applications in which a two-dimensional MEMS optical mirror array may be useful. Optical switches, for example, are the essential part of a DWDM (dense wavelength division multiplexing) optical network, and MEMS (micro-electro-mechanical system) optical switches provide a transparent, scalable, compact, low-cost, high performance solution. For MEMS optical switches, two general approaches are the binary 2-D switches and the analog 3-D switches. The 3-D approach has better scalability; however, the structure and control are more complicated. Gimbals of 2-degrees of freedom (typically for out-of-plane rotational motions) are needed for MEMS 3-D switches.
Device Issues: Electrostatic actuation devices are commonly used in MEMS because of the simplicity of this approach. Parallel plate, interdigitated and some hybrid designs are used for these actuators. Interdigitated electrode actuators have the advantage of small starting voltage and position-independent driving force, which is easier to control. To implement interdigitated electrode structures for an out-of-plane motion requires two electrode layers in a surface MEMS process. Motion in another out-of-plane orthogonal direction can cause undesirable electrostatic force components, contact of opposite electrodes and short circuit. To implement another out-of-plane motion, additional layers are needed. The layers should be adequately thick for the required output force generated by the electrodes, structure stiffness and robustness. Also, the payload (e.g., mirror) is typically a fraction of a millimeter in lateral size and a few degrees of out-of-plane rotational motion will induce a few tens of micrometer vertical motion at the edges. In addition, for purposes of electrical efficiency it typically is necessary to have small gaps between the electrodes to increase the torque output by providing high field intensity (everything else being the same), with the result that the electrodes are closer together and more likely to short circuit or come in contact. Thus, the implementation using interdigitated electrodes will require a multiple-layer high-aspect-ratio MEMS structures.
Process technology restriction: Surface micro-machining, bulk micro-machining and their combinations are used to implement MEMS 3-D switches. Bulk micro-machining can produce high-aspect ratio structures; however, their capability to implement multiple layers is limited. Surface micro-machining can produce multiple-layer structures; however, their structure thickness-to-width aspect ratio and feature resolution are generally limited, except in a LIGA-like process. (LIGA is an acronym in German meaning lithography electroplating and molding.) These high-aspect ratio surface micro-machining processes developed recently have for the most part been restricted to single layer applications. The fabrication of surface MEMS with high-aspect ratio layers requires minimizing the number of layers to be more manufacturable.
Optical system issue: For application in an optical networking system, the MEMS chip for mirror arrays and each beam-steering mirror in the array needs to be of a certain size decided by the optical path length and tolerable insertion loss, etc. The optical path is in turn decided by the port count (number of mirrors and their pitch) and actuator scanning ranges. If the overall optical system size can be scaled down, the optical path is short and the beam size of the gaussian beam from a single-mode fiber at the location of the steering mirror can be small. This allows the mirror to be made small, which in turn means the array can be more compact with the result that the optical path can be shorter. A large mirror with a small mirror pitch (large fill factor) thus gives the best system performance. The other requirement is that the optical signals need to be equalized in the DWDM system to minimize errors and processing times at the detection end.
Therefore, there is a need for a MEMS optical switch that meets the device, process, and optical system criteria described above.